JP 2007-74273 A discloses a coherent optical transmission method capable of transmitting multi-level signals. The method simultaneously transmits two signals having correlation through independent channels and offsets phase fluctuations of light sources. A reference code is transmitted by the one of optical carrier waves, and a multi-level signal where a to-be-transmitted data signal is superimposed on the reference code is transmitted by the other of the optical carrier waves. In addition, optical heterodyne detection or optical self-homodyne detection is performed by using two optical carrier waves included in a received optical signal, so that coherent optical transmission is performed.
JP 2005-6017 A discloses an optical wireless communication system which space-division multiplexes optical signals and transmits the multiplexed signal between a transmission apparatus and a reception apparatus.